Vacuum process chambers generally include a mounting chuck, for example, an electrostatic chuck that includes an electrode covered by a dielectric. In use, a DC voltage is applied to the electrode to generate an electrostatic clamping force to hold a substrate in place on the electrostatic chuck. The substrate may be, for example, a semiconductor wafer or a dielectric sheet. The force used to hold the substrate on the electrostatic chuck, known as a chucking force, is determined by the DC voltage applied to the chuck and by the distribution of electrostatic charge on the dielectric surface while the DC voltage is applied. The inventors have observed that the distribution of electrostatic charge on the dielectric surface, also referred to as a surface charge profile, impacts the subsequent process of discharging the substrate when the DC voltage is removed and the substrate is released from the mounting chuck, known as de-chucking. The presence of residual charge on the dielectric surface as a result of the incomplete discharging of the electrostatic charge may cause a substrate to break during de-chucking or may trap moisture or other chemicals on the dielectric surface that will impact subsequent processes in the process chamber.
Thus, the inventors have provided apparatus and methods to monitor or measure the electrostatic charge distribution on a dielectric surface of a chuck.